Power inverters have been used in variable-speed, constant-frequency power generating systems to convert DC power on a DC link into AC power for energizing one or more AC loads. Typically, such inverters include switches, such as transistors, which are operated by a control in a pulse-width modulated (PWM) mode to produce a PWM waveform comprising a series of pulses and notches. The waveform is converted into a sinusoidal output waveform by a filter which is coupled to the inverter output.
Ideally, the control should operate the inverter switches so that no DC power is produced in the output. However, operating conditions may cause DC content to be produced in the output of the inverter. This DC content can lead to undesirable consequences when the loads supplied by the inverter cannot tolerate same.
Roe et al. U.S. Pat. No. 4,882,120, assigned in the assignee of the instant application, discloses a DC content control for an inverter which detects the magnitude of a DC component in the inverter AC output power and adjusts the time of selected rising and falling edges of switch control waveforms stored in a memory to reduce the magnitude of the DC component. The control utilizes a finite state machine together with timers, logic gates and flip-flops to adjust the rising and/or falling edges.
Sato U.S. Pat. No. 4,729,082 discloses a control for a power converter which converts between AC and DC power in a bidirectional manner. In order to eliminate the DC component of current on the AC side of the power converter, the control operates the inverter to produce a DC voltage on the AC side which opposes the direct current component. The DC voltage is produced in one embodiment by shifting a half-cycle of the AC output waveform by a phase displacement which causes a DC component to be produced in the phase output. There is no clear description in this patent, however, as to how or by what means this is accomplished.